In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters and images. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using a plain paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate.
JP 9-217032A discloses a recording solution containing a dispersion of an organic pigment and a rosin emulsion in an aqueous liquid. In JP 9-217032A, although it is described that an aqueous resin may be mixed in the recording solution, there are no descriptions concerning colorant-containing polymer particles.
JP 10-140060A discloses an ink containing a colorant, a polyamide resin (film-forming resin), a tackifier resin such as a rosin ester, and an organic solvent. JP 2004-204103A discloses a pigment derivative containing a basic group, a polymer containing an ethylenically unsaturated monomer having a rosin skeleton as a polymerizable component, and a pigment-containing composition. However, the inks in these prior arts are of a non-aqueous type, and further the colorant is not incorporated in the resin.
JP 2003-138170A discloses a water-based ink containing an organic pigment, a rosin-modified water-soluble acrylic resin and water. However, the resin is water-soluble resins, and further the colorant is not incorporated in the resin.
JP 2004-277448A discloses a water-based ink containing a dispersion of a pigment being incorporated in a water-insoluble resin to render the pigment water-dispersible, resin fine particles containing a rosin, etc., a humectant, a penetrant and water. The pigment-containing polymer is an acrylic random copolymer and contains no rosin as a constitutional unit thereof.
The above conventional non-aqueous or water-based inks have been improved in properties thereof to some extent, but have failed to sufficiently suppress occurrence of unevenness or non-uniformity of colors upon printing with a high optical density.